In general, a single phase induction motor includes a stator wound around with a main coil and a sub-coil which are physically spaced 90° apart from each other, and a supply power is applied directly to the main coil, while indirectly (i.e., via a capacitor and a switch) to the sub-coil. This is because the single phase induction motor would not start even if a voltage is applied to the main coil. Therefore, a starting device such as the sub-coil is needed to create a rotor system at the stator, thereby starting or actuating the rotor.
There are many types of starting devices, for example, split phase start type, shaded coil start type, capacitor start type, repulsion start type, etc.
An capacitor start-type single phase induction motor is described as an example of a single phase induction motor, with reference to FIG. 1 and FIG. 2.
FIG. 1 illustrates a stator 10 and a rotor 20 in a conventional single phase induction motor, and FIG. 2 illustrates a simple circuit having a rotor coil and a stator coil.
When a main coil 12 is the only coil wound around the stator 10, only an alternating magnetic field is produced by the stator 10 and thus the rotor 20 does not start. However, when a sub-coil 14 is also wound around the stator to produce a rotating magnetic field whereby the rotor starts running or rotating in a certain direction. That is to say, the rotating magnetic field generates a starting torque.
Meanwhile, a capacitor 15 causes a phase delay of current being applied to the sub-coil 14 to generate a starting torque through the interaction with the main coil 12. Once started, if there is not going to be any change in a load, the rotor keeps rotating even if the sub-coil is not fed with power. Therefore, once the rotor started and keeps running at certain RPM or higher, it is all right to stop the power supply to the sub-coil. However, if the load is variable, a starting torque is needed. In this case, the sub-coil must always be provided with power through the capacitor.
On the contrary, a three-phase induction motor where a rotation system is easily created even by winding only the main coil around a stator, there is no need to wind the aforementioned sub-coil around the stator. In other words, a separate starting device is not necessary for the three-phase induction motor.
However, the single phase induction motor offers a competitive advantage over others in terms of price in that it does not require an inverter drive component of a BLDC (brushless DC) motor or a reluctance motor and can start up with the help of a common single phase power source.
Referring to FIG. 1 and FIG. 2, the detail description of the general single phase induction motor will be followed.
The stator 10 has a hollow interior space, an inner periphery of which is provided with a plurality of teeth 11 arranged at a predetermined angle interval, each being protruded inwardly in a radius direction and each being wound with the main coil 12 to have N-polarity or S-polarity at the application of a primary current.
An insulator (not shown) is provided between each of the teeth 11 and the main coil 12 to insulate between the tooth and the main boil and to facilitate the winding of the main coil.
The stator 10 also includes the sub-coil 14 that is wound physically spaced apart from the main coil 12 at a predetermined angle to produce a rotating magnetic field when current is applied thereto. Of course, the sub-coil is wound around the teeth 11 via the insulator, and the main coil 12 and the sub-coil 14 together are called a stator coil or simply a coil.
The coils 12 and 14 are connected to a single phase power source, in parallel to each other. Moreover, the sub-coil is serially connected to the capacitor 15. Although not shown, the capacitor may be connected selectively to the power source through a switch.
Generally, a squirrel cage rotor is used most in the field, so the rotor 20 shown in FIGS. 1 and 2 represent the squirrel cage rotor.
The rotor 20 is formed by stacking a plurality of identically shaped steel sheets, each steel sheet having a plurality of slots 21 formed at a predetermined angle interval along the outer circumference at a predetermined radial position from the core. In addition, the rotor 20 includes conductive bars 22 inserted into the slots 21 of the rotor core, and the conductive bar is usually made out of copper or aluminum.
In order to cause an electrical short through the conductive bars, both ends of the squirrel cage rotor are connected by an end ring (not shown in FIGS. 1 and 2, referred to FIGS. 11 and 12 later), and the end ring is typically formed by an aluminum die casting process. That is, the conductive bar 22 and the end ring are integrated through aluminum die casting, and the end ring is formed at the upper and lower portions of the rotor core, respectively. Meanwhile, an axial bore 24 is formed in the core of the rotor 20, and a shaft (not shown) transferring torque of the rotor to other components is press fitted into the axial bore such that the rotor and the shaft can rotate in one unit.
According to how the single phase induction motor with the above configuration works, when power is applied to the coil, an induced current is produced in the conductive bars 22, through which an induction torque is generated to rotate the motor. In this case, however, a loss occurs in a the conductive bars 22, the loss is so called a conductive bar loss. Because of the conductive bar loss, there is a limitation in enhancing the efficiency of a motor with a fixed size. Therefore, single phase induction motors were not suitable, sometimes useless, for high-efficiency work.
Besides, the rotor 20 gets hot because of the conductive bar loss, and such a temperature change of the rotor in turn made the loss even higher. In other words, the conductive bar loss gets worse as the temperature of the rotor increases. This remained as another limitation in improving the efficiency of a motor at high temperature.
In the meantime, it is known that the single phase induction motor, by its nature, should always run slower than a preset synchronous speed, to be able to produce an induced torque. This is because, theoretically, the amount of torque of the single phase induction motor stays zero at the synchronous speed, and it tends to increase at low RPMs.
In short, a problem arises in the single phase induction motor in relation to the control of the motor in response to a change in the motor load since the speed of the motor shaft, i.e., the motor speed, varies with the load on the motor, i.e., the load on the motor shaft.